Auto-decode columns declared as JSON in output modes (issue #579)

Columns declared with type JSON now decode automatically wherever nested
JSON is already supported (query/rows/search/memory output, and the
Python API), without needing --json-cols. Uses sqlite3's built-in
detect_types=PARSE_DECLTYPES + a registered JSON converter, so it works
transparently through joins and aliases. Flat formats (CSV/TSV/table/raw)
re-serialize decoded values back to JSON text instead of a Python repr.
Also adds "JSON" as a first-class column type for create()/add_column()/
transform(), and fixes column_affinity("JSON") so transform() doesn't
rewrite JSON columns as REAL.

tests/test_create.py

@@ -375,6 +375,12 @@ def test_create_error_if_invalid_self_referential_foreign_keys(fresh_db):
             'CREATE TABLE "dogs" (\n   "name" TEXT\n, "float" FLOAT)',
         ),
         ("blob", "blob", None, 'CREATE TABLE "dogs" (\n   "name" TEXT\n, "blob" BLOB)'),
+        (
+            "friends",
+            "JSON",
+            None,
+            'CREATE TABLE "dogs" (\n   "name" TEXT\n, "friends" JSON)',
+        ),
         (
             "default_str",
             None,